2. Field of the Invention
The present invention relates to the production of nitrogen, and more particularly to a nitrogen producing method and apparatus which are well suited to produce superpure nitrogen with a low impurity content.
2. Description of the Prior Art
In recent years, superpure nitrogen containing very little impurities has been increasingly demanded for use in, for example, the fabrication of submicron LSIs. A nitrogen producing apparatus which has been generally known is disclosed in the Official Gazette of Japanese Patent Application Laid-Open No. 84888/1979 or No. 225568/1986 or U.S. Pat. No. 4,617,040. In such apparatus feed air is cooled to a low temperature and is thereafter subjected to rectifying separation by a rectifying column. Then the separated that nitrogen gas is taken from the upper part of a rectifying column Then, the separated nitrogen gas has its temperature restored to the normal temperature by an air heat exchanger, and the resulting gas is obtained as product nitrogen.
Further, in such nitrogen producing apparatus, carbon monoxide and hydrogen are removed by catalytic burning. Remaining hydrogen is also separated and removed by gas blows from the top of the rectifying column and particles are removed by inserting a filter in a liquid nitrogen line.
The nitrogen producing apparatus in the prior art does not take into consideration the need to remove impurities such as moisture. Accordingly, it has a drawback that the moisture and other outgases contained in the heat exchanger, piping, valves etc. mix into the nitrogen product in the process of removing the impurities, such as oxygen, carbon dioxide, hydrocarbon and moisture, to the order of ppb by means of the rectifying column, drawing the nitrogen gas at the low temperature out of the upper part of the rectifying column, and thereafter restoring the temperature to the normal temperature by means of the air heat exchanger.
Moreover, the construction of the nitrogen producing apparatus in the prior art does not take into consideration the need to remove low-boiling impurities such as carbon monoxide and hydrogen to the order of ppb. In this regard, the removal of the carbon monoxide resulting from on the catalytic burning is due to the short lifetime of a catalyst which is attributed to the deterioration of the catalyst. Besides, in the separation and removal of the hydrogen, only the gas blows from the top of the rectifying column are performed, and the number of rectifying stages which is necessary for the rectifying separation are not provided, so that the carbon monoxide and the hydrogen cannot be removed to an order of several ppb. In addition, regarding the removal of the particles, particles having diameters of 0.1 .mu.m cannot be reduced to several particles/ft.sup.3 or less because the filter is not incorporated in the liquid nitrogen line.